Chronology and processes of fluvial terrace formation in northeastern Kinki district, southwest Japan, based on cryptotephra analysis

摘要

Kinki district is a tectonically active area in southwest Japan. This area is located far from active volcanoes, and eolian deposits covering fluvial terrace deposits are not as thick as those in Kanto district. Thus, terrace chronology and processes of terrace formation previously were poorly established in Kinki district. Cryptotephra analysis identified invisible tephra horizons in eolian deposits covering fluvial terrace deposits in Takashima, Sekigahara and Inabe regions, including the Kikai-Akahoya tephra (K-Ah: 7.3 ka), Aira-Tn tephra (AT: 26-29 ka) and Kikai-Tozurahara tephra (K-Tz: 95 ka) horizons. Taisanjino 1 terrace in Takashima and Md1 terrace in Inabe were formed during Marine Isotope Stage (MIS) 5e, and Nakano terrace in Takashima, L2 terrace in Sekigahara and L2 terrace in Inabe were formed during MIS 2, based on their ages and geomorphic features. Aggradational terraces formed during MIS 2 exhibit similar geomorphic features among the three regions, although these regions have different base-levels of erosion and active fault movements. This means that the terrace formation during MIS 2 was mainly affected by climate changes, not by base-level changes and active fault movements. The main factor of terrace formation during glacial periods was the decrease in precipitation and water discharge due to climate changes, because the study area was not affected by glacial and periglacial processes during MIS 2. This indicates that aggradational terraces formed during glacial periods not only in glacial and periglacial areas but also in non-glacial and non-periglacial areas. From processes of terrace formation during MIS 5e, the tectonic base-level descent was important for terrace formation during interglacial periods, because terraces were formed during MIS 5e by active fault movements in the Takashima region despite small lake-level changes. Uplift rates of the Kamidera and Kuwana faults were 1.08 mm/y and >0.5 mm/ y, respectively, based on the height of terrace surfaces formed during MIS 5e.